VITAMIN C AT 120K: EXPERIMENTAL AND THEORETICAL STUDY OF THE CHARGE DENSITY P. Ugliengo⁺ R. Bianchi*B. Civalleri⁺ C. Roetti⁺ and D. Viterbo^{+ +}Dip. di Chimica IFM, Torino, and *CSRSRC, CNR, Milano, Italy.

The experimental charge density of Vitamin C, obtained from low temperature diffraction data, has been compared with that obtained from quantum-mechanical calculations performed both on isolated molecules and on crystals.

Vitamin C is an extremely interesting molecule, not only for its important biological properties, but also for its crystal structure in which the formation of several hydrogen bonds plays an important role. We have collected X-ray diffraction data at -153deg.C up to sin[[theta]]/[[lambda]] ~1. The crystals do not undergo any phase transformation and the space group remains P2₁ with two independent molecules in the asymmetric unit. In the final difference Fourier map bonding non-spherical electron density was clearly indicated. A study by means of a multipole expansion of the electron density has been performed and the experimental electrostatic potential obtained in this way has been compared with that computed by ab-initio calculations on the isolated molecule, in order to attain a better understanding of the effects of the crystal packing on the charge distribution. Other one-electron properties, such as the dipole moment, have also been evaluated and compared. A complete conformational analysis of the isolated molecule has also been carried and the most stable structure has a folded conformation with a strong intramolecular hydrogen bond. We have also computed the electrostatic potential by means of a crystal-orbital approach using the program CRYSTAL [R. Dovesi, M. Caus, R. Orlando, C. Roetti and V.R. Saunders, J. Chem. Phys. 92, 7402 (1990)] with the MINI-1 basis set. The maps are in good agreement with the experimental ones. Because our system is relatively large, so far we have not been able to perform calculations with a more extended basis set, but we are presently checking our results with those obtained on smaller systems, such as urea and formamide.